In a case where a motor is locked while the motor for driving a mechanism, such as a slide mechanism, a lift mechanism and the like, is being energized, an excessive electric current may flow through the motor, which may result in a burnout of the motor. In order to prevent the motor from being burned, i.e. in order to protect the motor, for example, the following devices are provided.
Disclosed in JP2003-289694A is a drive switching device, in which a motor current flowing through a motor and a time period for the motor being energized (i.e. an energizing time) since the motor has started are detected, so that a heating temperature of the motor is estimated on the basis of the motor current and the energizing time. According to the drive switching device disclosed in JP2003-289694A, electrification of the motor is restricted (i.e. operation input via an operation switch is allowed or prohibited) on the basis of the estimated heating temperature.
Disclosed in JP2007-43835A is a motor controller, which calculates an estimated temperature of a motor, memorizes the calculated estimated temperature and calculates a temperature increasing value of the estimated temperature on the basis of an operating condition of the motor while being energized in order to update the estimated temperature by adding the calculated temperature increasing value to the memorized estimated temperature. In a case where the updated estimated temperature exceeds a predetermined temperature, a burning protection of the motor is executed. In the calculation of the temperature increasing value, plural temperature increasing values are calculated by using plural temperature increase estimating equations and the greatest temperature increasing value among the calculated temperature increasing values is selected as the most appropriate temperature increasing value.
According to the drive switching device disclosed in JP2003-289694A and the motor controller disclosed in JP2007-43835A, the temperature of the motor is estimated on the basis of the energizing time, the electric current and the like, so that the electrification of the motor is restricted in the case where the estimated temperature exceeds a preliminarily set limit value (a predetermined temperature). According to the drive switching device disclosed in JP2003-289694A and the motor controller disclosed in JP2007-43835A, the same limit value is used for both a case where the motor is used under a low-temperature environment and a case where the motor is used under a high-temperature environment. In order to ensure a safety of the motor, the limit value is set in consideration of the high-temperature environment. Therefore, in a case where the limit value, which is set in the consideration of the high-temperature environment, is used for the motor, which is used under the low-temperature environment, the electrification of the motor is restricted even at a condition well below a burnout of the motor actually occurs, and full advantage of the motor may not be used.
A need thus exists to provide a motor control device which is not susceptible to the drawback mentioned above.